Can someone explain this maths answer to me?

[MrsFogi]

I'm just marking dd's maths. The question is "Show that 113 is a prime number". The answer in the book is "113 is less than 120, ends in a 3 and doesn't dive by 3 or 7, so 113 is a prime number". Can someone enlighten me why we look to 120 and why we divide by 3 and 7 to prove it?

I think they mean it's under 121 so you need to check whether it divides by any prime up to but not including 11. It ends in a 3 so isn't divisible by 2 or 5. That leaves division by 3 and by 7 to check.

Prime numbers are only divisible by 1, and themselves. So it's showing you ways to see if it is divisible by other numbers.